182 EVAPORATION. 



abundantly does the air supply them with nature's 

 most abundant, most refreshing, and most valuable 

 production. If you would know the real value of 

 water, ask a man when he is stretched on his couch 

 in the heat of a fever, and when his throat is in- 

 flamed and swollen so that it will not do its office ; 

 or if he cannot answer, then ask him who sinks down 

 under the ardours of the mid-day heat, on the wide 

 and burning sand of Sahara, at many leagues' distance 

 from the little dingy pool and the overshadowing 

 palms : question him as to the value of water, and, 

 though the charter of the world's wealth were in his 

 keeping, he would cheerfully give it for one little 

 cup, or even that he were sitting on the brink of one 

 of those stagnant ditches which we shun. 



As we do not see the particles of the atmosphere 

 as a whole, the particles of its two chief ingredients, 

 the oxygen and the hydrogen, or the particles of 

 water which it takes up in the process of evapora- 

 tion, we cannot know the nature of the agency by 

 which any of these are held together. The cohesion 

 of particles in the entire substance, as air, is indeed 

 not only small, but absolutely negative, and entirely 

 obedient to the action of heat ; and not only that, 

 but if air is let into a larger space upon which there 

 is no pressure it will expand ; and cool, that is, be- 

 come sensibly cold, or abstract heat from other sub- 

 stances as it expands. And when the quantity of it 

 in a close vessel is diminished by pumping a por- 

 tion of it out, and water is placed in the vessel, and 

 some substance is also placed in it which has more 

 attraction for water than the air has, and which in 

 consequence drinks up the vapour of the water as 

 soon as it is formed, the remaining air in the vessel 

 will become so cold that the water will be frozen 

 into a cake of ice, even though the apparatus be in 

 a warm room. 



That simple experiment throws some light upon 

 he very general and important process of evapo- 



